Gesundheit is the German word for health. When somebody sneezes Germans reply by saying "Gesundheit!" to wish them good health. This expression has also found its way into the English language with the first recorded examples appearing in the 1910s. The expression was probably common even earlier in German-speaking areas of America and spread from there.
There are different theories about the origin of this phrase. One likely explanation is that the expression stems from the Dark Ages when the Plague and other diseases were threatening Europe. In this case the one saying "Gesundheit" was actually wishing for good health for himself and trying not to catch anything from the one who had sneezed (including demons perhaps).
